Natural products have a proven record of providing a significant fraction, either directly or as lead compounds, of human medicines. Among natural products, the terpenoids (isoprenoids) stand out as being the largest class (>50,000 already known), with the 20-carbon diterpenoids targeted here forming a significant fraction of these (>12,000 known). Indeed, the extensive diversification of diterpenoids indicates that the manifold structures that can be formed from this C20 backbone provide a rich source of biological activity, and a number of these natural products are used as pharmaceuticals (e.g., the anti- cancer paclitaxel/TaxolTM and anti-biotic mutilins) or research tools (e.g., the protein kinase C activating and tumor promoting phorbol esters, and adenylate cyclase activator forskolin). Conversely, we have recently discovered that the human pathogen Mycobacterium tuberculosis utilizes a diterpenoid, isotuberculosinol, to modulate our initial immune response to infection. Accordingly, we propose here to continue our productive investigations of the enzymes, from both plants and microbes, that catalyze formation of the underlying hydrocarbon skeletal structures. Specifically, we will build on our findings from the previous grant period, which includes not only the discovery of isotuberculosinol, but elucidation of some of the first diterpene synthase crystal structures as well. This proposal then advances our long- term goal of engineering enzymes and metabolic pathways for the production of targeted libraries and specific individual terpenoid 'natural' products for pharmaceutical investigation and use. In particular, the objective of this proposal is a detailed structure-function investigation of the synthases/cyclases that catalyze formation of the hydrocarbon skeletons underlying the structural diversity of diterpenoid natural products. These enzymes catalyze complex electrophilic reactions that form new carbon-carbon bonds, and play important roles in initiating diterpenoid biosynthesis. Accordingly, our studies have implications not only for providing increased access to such natural products, but also insights into potential drug targets, specifically in the widespread pathogen M. tuberculosis.